Nucleophilic Addition to Olefins. 22. Kinetics of Hydrolysis of the Piperidine and Morpholine Adducts of Benzylideneacetylacetone in 50percent Me2SO-50percent Water

Article abstract of DOI:10.1021/jo00244a029

Benzylideneacetylacetone reacts with piperidine and morpholine to form a pH-dependent mixture of anionic adducts, PhCH(R2N)C(COCH3)2^- (T_A^-), and zwitterionic adducts, PhCH(R2NH⁺)C(COCH3)2^- (T_A^+/-), which slowly hydrolyze to benzaldehyde, acetylacetone (or its anion), and R2NH.A kinetic study of this hydrolysis in 50percent Me2SO-50percent water shows that at high pH carbon protonation of T_A^-, to form PhCH(R2N)CH(COCH3)2 (T_A⁰), is rate-limiting while at intermediate pH carbon protonation of T_A^+/-, to form PhCH(R2NH⁺)CH(COCH3)2 (T_A⁺), is co-rate-limiting with intramolecular proton transfer, T_A^+/- -> T_A⁰ (Scheme I).At low pH the enol form of T_A⁺ becomes an important intermediate (Scheme II).Rate and equilibrium constants for the various elementary steps leading from T_A^+/- and T_A^- to T_A⁰ were determined or estimated and a lower limit for the rate constant of breakdown of T_A⁰ into PhCH=N⁺R2 and CH(COCH3)2^- was estimated.Comparison of the kinetic behavior of benzylideneacetylacetone adducts with that of amine adducts of four other PhCH=CXY-type olefins shows both similarities and important differences which are discussed in detail.It appears that there are two major factors that determine whether formation of T_A⁰ or its breakdown is rate-limiting.One is crowding, which enhances breakdown and slows proton transfer.The other is the sensitivity of the intrinsic barriers to resonance effects in the carbanion that is formed in the breakdown of T_A⁰ on the one hand and that which is formed in the deprotonation of T_A⁰ on the other hand.The smaller sensitivity to this factor in the breakdown reaction tends to make breakdown rate-limiting for systems where resonance in the carbanion is modest and to make proton-transfer rate-limiting where this resonance is strong.